The present invention is related to wireless networks, and in particular, to using decision feedback equalizers in a receiver of OFDM signals.
Wireless networks are becoming more widespread. The IEEE 802.11 wireless local area standard, for example, describes a popular wireless local area network standard. Different variants of the standard use different modulation methods. The IEEE 802.11a variant and the OFDM part of the 802.11g variants use a multicarrier method such as Orthogonal Frequency Division Multiplexing (OFDM) which has the feature of being effective in the presence of multipath and the delay spread it causes.
In a typical OFDM modulation method, delay spread is dealt with by a cyclic extension that “hides” the smeared-out energy (the delay spread). However, when the delay spread is long, e.g., energy in the multipath “echoes” of the one OFDM symbol (called delays) has not tapered to near-zero after the cyclic extension length at the next OFDM symbol, then the long delays then affect the next OFDM symbol and degrade the signal quality, e.g., as measured by the error vector magnitude (EVM) of the next OFDM symbol, and thus the packet error rate (PER) of the communication. We call this effect inter-OFDM-symbol interference. This effect occurs at a lower delay spread as the data rate increases. For example, in a typical IEEE 802.11a system, the inventors have found that communication at 6 Mbit/s breaks down with delay spreads of more than about 400 ns, while 54 Mbit/s communication breaks down for delay spreads greater than about 125 ns RMS.
There thus is a need to improve the inter-OFDM-symbol interference performance of OFDM receivers in the presence of long delay spreads.
In particular, there is a need in the art for a method and an apparatus that remove inter-OFDM-symbol interference caused by long delay spread to provide high data rates, e.g., almost independently of the length of the delay spread.
In single carrier communication links, methods and apparatuses are known to deal with delay spread and the effects it has on the channel impulse response, carrier-to-interference ratio (CIR), dispersion, frequency selectivity, and intersymbol interference (ISI), include using a linear equalizer (equalizer) and using a decision feedback equalizer (DFE).
In OFDM receivers, the problem to be solved is not quite the standard equalization problem. In OFDM, delays up to the cyclic extension length provide a desirable or benign contribution, since the much of the energy of the spread is captured by the discrete Fourier transform (DFT) operation typically implemented in an OFDM receiver by a fast Fourier transform (FFT) and most of the remaining energy occurs up to the cyclic extension and so is not input to the DFT operation and hence does not cause inter-OFDM-symbol interference. Delay spread beyond the cyclic extension decreases performance.
Thus there is a need in the art for an equalizer that removes distant delays that lead to inter-OFDM-symbol interference, but not short delays.
A linear equalizer comprising a feedforward filter whose filter coefficients are the inverse of the channel impulse response is known. For the OFDM variants of the IEEE 802.11 standard, an OFDM packet includes only two long symbols and these directly abut data symbols. Thus, if a linear equalizer is to be used, there is typically not sufficient time to adapt to the proper feedforward filter coefficients. Thus, these coefficients would need to be computed directly, and very fast. Using a linear equalizer and estimating its coefficients by inverting the channel impulse response is computationally messy. Therefore, a decision feedback equalizer that has a trivial feed-forward filter is more desirable.
Thus there is a need in the art for a decision feedback equalizer that removes distant delays that lead to inter-OFDM-symbol interference, but that does not necessarily remove short delays.